The present invention relates to membrane switches and more particularly to a flat high temperature membrane switch display window for use in association with appliances, such as electric ranges.
Several types of appliances, such as ranges, cooktops and the like have control panels and membrane switches with electronic displays. These membrane switches and displays have become popular in many later model units. However, problems arise because of the cooktop environment and the elevated temperatures associated therewith. Thus, the control panel on the appliance experiences heating at levels above ambient room temperature.
Currently, membrane switches and displays are made by adhering multiple layers of thin plastic together and then bonding that assembly to a rigid substructure. In applications where some type of display is mounted behind the switch assembly, it is common practice to put a hole in the substructure and in all the switch layers except the outermost layer (the overlay layer). The overlay layer is the layer the user sees, and it forms a smooth, easily cleaned surface. The display is viewed through a transparent area of the overlay (a display window). The display window is not adhered to anything because all of the adhesives currently available will over time become cloudy and obstruct the view of the display.
The common overlay materials, for example, polyester and polycarbonate, are heat sensitive and cause distortion of the display window at relatively low temperatures on many cooking appliances. This problem relates to the "perceived quality" of the appliance. The user finds this distortion unappealing and unacceptable in practice. Thus, it is apparent that maintaining fiat display windows on appliances, such as open door boilers, in vent areas and in areas with two rear burners on a free standing range is a constant design problem requiting additional tests, design time and money.
In the past, the portion of the overlay of the membrane switch covering the display consisted of a single layer of transparent, flexible material. The overlay thus contained a volume of air in the space between it and the display. When this volume of air becomes heated due to increased cooktop of oven temperatures, the volume of air expands, and the overlay softens from the heat. Thus, when exposed to the heat, the overlay distorts as a result of the pressure of the increased volume of heated air. Thus, the membrane would no longer have an attractive flat appearance; it would bubble and appear uneven making the display illegible.
U.S. Pat. No. 4,611,261 relates to an electronic calculator which has a membrane overlay covering the keypad and display. Also, beneath a membrane in the display area is a rigid polarizing plate that is not fastened to the membrane. Thus, no means for fastening the plate to the membrane is provided in this device.
U.S. Pat. No. 4,901,074 relates to a membrane switch on a cooking appliance. A layer of glass is provided on the outside of the control keypad and the display to resist heat.